Manufacture of mosaic screens such as are utilized in television transmission tubes



May 15, 1956 H. CASSMAN 2,745,772 *MANUFACTURE OF MOSAIC SCREENS SUCH ASARE UTILIZED IN TELEVISION TRANSMISSION TUBES Filed Feb. 15, 1951"Ill/IIIIIIIIIIIIIIIIIIIIIIIIIIllfl I I 'I/lI/IIIIIIII/ lnvonfr HARRYCASSMAN nited States i-iarry Cassman, Harlington, England, assignor toElectric & Musical Industries Limited, Hayes, England, a company ofGreat Britain Application February 15, 1951, Serial No. 211,013

Claims priority, application Great Britain February 23, 1950 4 Claims.(Cl. 117-210) This invention relates to mosaic electrodes such as areutilised in television pick-up tubes and to a method of producing suchelectrodes.

A known form of mosaic screen utilised in television pick-up tubescomprises a sheet of dielectric material, such as glass or mica, to oneside of which is applied a multiplicity of discrete photo-electricallysensitised conductive elements. Said elements are applied by evaporatinga metal, such as antimony or silver, on to the dielectric sheet throughthe interstices of a fine mesh mounted to function as a stencil in frontof the sheet. It has been found that, in practice, the degree ofinsulation between the discrete conductive elements is important,especially when the mosaic screen is intended for use in a televisionpick-up tube adapted to operate with cathode potential stabilisation andthat there is an optimum value of conductivity between the elements. Itis known that the sensitisation of the discrete conductive elements isusually efiected by exposing the conductive elements to caesium vapourand then to oxygen, and the degree of insulation may be modified inpractice by removing the stencil mesh prior to the admission of caesiumvapour, so that the caesium and, later, the oxygen have access not onlyto the conductive elements but to the areas of the dielectric sheetexposed between them. In one method, the exposure to caesium vapour iscontinued until the insulation has become lower than eventually requiredand then the subsequent exposure to oxygen, which tends to restore theinsulation, is continued until the photo-sensitivity and definition ofthe mosaic screen have maximum values. However, this method has thedisadvantage of being highly critical, especially as regards the excessof caesium deposited, and it is found that if too great an excess ofcaesium is allowed to be present on the mosaic screen, the mosaic screentends to have poor definition and produce fuzzy pictures, whereas if theexposure to caesium is insufiicient the mosaic screen, if it is utilisedin a cathode potential stabilised tube, manifests an effect which istermed the photographic effect due to excessive insulation between theconductive elements.

The object of the present invention is to mitigate the above-mentioneddisadvantages.

According to the present invention, a method of manufacturing a mosaicelectrode comprises the steps of coating one surface of a dielectricsupport with a caesium resistant film of metal oxide (as hereinafterdefined), mounting a stencil mesh in front of the coated surface of saidsupport, depositing metal through said stencil to provide a multiplicityof conductive elements on said coating, removing the stencil mesh, andthereafter exposing the conductive elements and the parts of the coatingbe tween said elements to caesium to sensitize said elements.

According to an aspect of the present invention there is provided amosaic electrode such as is utilised in television pick-up tubes,comprising a support of dielectric material, a coating of a magnesiumoxide applied to said atent O 2,745,772 Patented May 15, 1956 support, amultiplicity of discrete conductive elements deposited on said coating,and a photo-electrically sensitising deposit comprising caesium andoxygen on said conductive elements and extending over areas of saidcoating exposed between said elements.

Where reference is made herein to a caesium resistant metal oxide, it isto be understood to mean a metal oxide which is not reduced by caesium,at least to any appreciable extent.

In order that the said invention may be clearly understood and readilycarried into effect, the same will now be more fully described withreference to the accompanying drawing, in which:

Figure 1 illustrates in diagrammatic cross-sectional view a fragment ofa mosaic electrode according to one example of the invention, and

Figure 2 illustrates a stage in the process of manufacturiug theelectrode illustrated in Figure 1.

Referring to the drawing, the mosaic electrode illustrated comprises adielectric support which is in the form of a thin transparent sheet, forexample of glass or mica, having a thickness of, say, between 0.001 and0.004 inch. One surface of the sheet 1 has applied to it a thinsubstantially transparent coating of magnesium oxide 2, and amultiplicity of minute discrete conductive elements, the so-calledmosaic elements, are deposited on the outer surface of the coating 2,three such elements denoted by the reference 3 being visible in thedrawing. The elements 3 are photo-electrically sensitised with caesiumand oxygen as hereinafter described and this has been denoted in thedrawing by a thin film 4 which, it will be observed, extends not onlyover the elements 3 but over the areas of the magnesium oxide coating 2which would otherwise be exposed between the elements 3. The othersurface of the sheet 1 is provided with a continuous substantiallytransparent metallic coating 5 which constitutes the signal plate forthe mosaic electrode. in known manner, the mosaic electrode beingintended for use in a television pick-up tube in which the optical imageto be televised is projected through the signal plate 5, the sheet 1 andthe magnesium oxide coating 2 on to the mosaic elements 3. i

In manufacturing the electrode illustrated in Figure l the coating 2 isformed by evaporating a thin continuous film of magnesium on to theappropriate surface of the sheet 1. The quantity of magnesium requiredis small and in practice it has been found that satisfactory results areobtained if magnesium is deposited to a thickness such as to reduce thelight transmission through the sheet 1 by between 50 and 60 per cent.The sheet 1 provided with a thin film of magnesium is then exposed toair to oxidise the magnesium, the film becoming effectively transparentas a result of oxidation, thus completing the formation of the coating2. Before forming the coating 2 the signal plate 5 may be applied on theother side of the sheet 1, but the signal plate may if desired beapplied at any other convenient time during the manufacturing process.After the sheet 1 has the coating 2 formed thereon the sheet is mountedin position in the envelope of the television pickup tube for which itis intended, and the mosaic elements 3 are deposited on the coating 2 inany suitable manner, for example by evaporating one or more metals, sayantimony, or bismuth and silver, through the interstices of a meshmounted to function as a stencil in front of the exposed surface of thecoating 2. This stage of the process is illustrated in Figure 2, afragment of the stencil mesh being indicated by the reference 6 whilethe arrow 7 denotes the trajectory of the evaporated metal or metals.The stencil is then removed and mosaic elements 3 and those areas of theoxide coating 2 exposed between the elements 3 are exposed to caesiumvapour. The caesium treatment, which is effected in a conventionalmanner, is

elements 3 is higher than is desirable in the completed electrode.During the exposure to caesium the electrode is baked in known manner ata temperature between 140 and 170 C. After cooling, the electrode isthen exposed to Oxygen while the pick-up tube in which it is mounted isconnected in a television camera circuit and the exposure to oxygen iscontinued until the insulation between the mosaic elements has anoptimum value as indicated by the photoelectric sensitivity anddefinition of the electrode being maxima. Observations of thephoto-sensitivity and definition are made by observing a picturereproduced from signals generated by the camera when an optical image ofcontrasting intensities is focused on the mosaic electrode in thepick-up tube.

It is found that television pick-up tubes with mosaic electrodesmanufactured as above-described are more stable in their performancethan tubes manufactured by the method described in the second paragraphof the specification. The mechanism of the improvement is not clear butthe presence of the magnesium oxide coating renders the performance ofthe electrode in some manner, apparently connected with the fact thatthe magnesium oxide is not reduced by caesium, less critical with regardto the amount of caesium deposited on the areas exposed. between theelements 3. Provided that the amount of caesium deposited on theelements 3 prior to the final oxidation is such that the maximum ofphotoelectric sensitivity is passed, as described, there is lesstendency for the mosaic electrode to cause fuzziness or to manifest thephotographic eifect in use. There is, moreover no substantial loss inphoto-sensitivity due to the presence of the magnesium oxide coating.

The invention is not confined to the employment of a coating ofmagnesium oxide and the coating may be formed of oxides of other metalsprovided that the oxide used is not reduced by caesium. For examplealuminium oxide and barium oxide may be used instead of magnesium oxide,and other refractory oxides may also be suitable.

What I claim is:

l. A mosaic electrode, such as is utilized in a television pick-up tube,comprising a sheet of dielectric material, a

coating of magnesium oxide applied to one surface of said sheet, amultiplicity of discrete conductive elements applied to said coating,and a photoelectrically sensitizing deposit of oxidised caesium on saidconductive elements, said magnesium oxide coating protecting said sheetof dielectric material from said caesium.

2. A method of manufacturing a mosaic electrode such as utilized intelevision pick-up tubes, comprising the steps of coating one surface ofa dielectric support with a caesium resistant metal oxide film, mountinga stencil mesh in front of the coated surface of said support,depositing metal through said stencil to provide a multiplicity ofconductive elements on said coating, moving the stencil mesh away fromsaid surface, and thereafter exposing the conductive elements and theparts of said coating between said elements to caesium and oxygen tosensitize said elements.

3. A method of manufacturing a mosaic electrode such as utilized intelevision pick-up tubes, comprising the steps of coating onesurface ofa dielectric support with a refractory oxide film, mounting a stencilmesh in front of the coated surface of said support, evaporating metalthrough said stencil mesh to deposit a multiplicity of conductiveelements on said coating, moving the stencil mesh away from saidsurface, and thereafter exposing the conductive elements and the partsof said coating between said elements to caesium and oxygen to sensitizesaid elements.

4. A method of manufacturing a mosaic electrode such as utilized intelevision pick-up tubes, comprising the steps of coating one surface ofa dielectric support with a film of metal oxide selected from the groupconsisting of magnesium oxide, aluminium oxide and barium oxide,mounting a stencil mesh in front of the coated surface of said support,evaporating metal through said stencil mesh to deposit a multiplicity ofconductive elements on said coating, moving the stencil mesh away fromsaid surface, and thereafter exposing the conductive elements and theparts of said coating between said elements to caesium and oxygen tosensitize said elements.

References Cited in the file of this patent UNITED STATES PATENTS2,171,224 Rose Aug. 29, 1939 2,198,327 Bandringa et al. Apr. 23, 19402,434,930 Johnson Jan. 27, 1948

2. A METHOD OF MANUFACTURING A MOSAIC ELECTRODE SUCH AS UTILIZED INTELEVISION PICK-UP TUBES, COMPRISING THE STEPS OF COATING ONE SURFACE OFA DIELECTRIC SUPPORT WITH A CAESIUM RESISTANT METAL OXIDE FILM, MOUNTINGA STENCIL MESH IN FRONT OF THE COATED SURFACE OF SAID SUPPORT,DEPOSITING METAL THROUGH SAID STENCIL TO PROVIDE A MULTIPLICITY OFCONDUCTIVE ELEMENTS ON SAID COATING, MOVING THE STENCIL MESH AWAY FROMSAID SURFACE, AND THEREAFTER EXPOSING THE CONDUCTIVE ELEMENTS AND THEPARTS OF THE SAID COATING BETWEEN SAID ELEMENTS TO CAESIUM AND OXYGEN TOSENSITIZE SAID ELEMENTS.